Dropping criminal charges occurs for many reasons, including a victim who changes his mind about the complaint; however, the prosecuting attorney is the person who decides to drop them. Other reasons prosecutors drop charges include a lack of a viable witness and evidence obtained illegally.
Some victims ask the prosecutor to drop the charges because they are afraid of the person the prosecutor is charging, or in domestic violence cases, the victim loves the person and wants to maintain a relationship with him. Victims also realize after their initial report to the police that they chose the wrong person. Victims who change their story substantially when asking for the dropping of charges sometimes face criminal charges themselves.
In order to prosecute a case, the prosecutor must have viable witnesses. Witnesses need to be convincing to the judge or jury. If the prosecutor feels the chances of conviction are poor, he sometimes drops the charges so he does not waste resources in the case.
In gathering evidence for a case, the police must follow protocol. They cannot search a home without a warrant or permission of the owner. If the prosecutor knows a judge is likely to rule the major evidence in a case as inadmissible, he drops the case for a lack of evidence.